The use of zeolitic adsorbents consisting of faujasite (FAU) zeolites of type X or Y comprising, besides sodium cations, barium, potassium or strontium ions, alone or mixed, for selectively adsorbing para-xylene in a mixture of aromatic hydrocarbons, is well known from the prior art.
U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020 and 3,663,638 show that zeolitic adsorbents comprising aluminosilicates based on sodium and barium (U.S. Pat. No. 3,960,774) or based on sodium, barium and potassium, are effective for separating para-xylene present in C8 aromatic cuts (cuts comprising aromatic hydrocarbons with 8 carbon atoms).
The adsorbents described in patent U.S. Pat. No. 3,878,127 are used as adsorption agents in liquid-phase processes, preferably of the simulated countercurrent type, similar to those described in patent U.S. Pat. No. 2,985,589, and which are applied to the C8 aromatic cuts, among others.
In the patents listed above, the zeolitic adsorbents are in the form of crystals in the powder state or in the form of agglomerates constituted predominantly of zeolite powder and up to 20 wt % of inert binder.
The FAU zeolites are usually synthesized by nucleation and crystallization of aluminosilicate gels. This synthesis leads to crystals (generally in the form of powder) whose use on an industrial scale is particularly difficult (large head losses during the operations). Therefore the agglomerated forms of these crystals are preferred, in the form of grains, spun yarn and other agglomerates, said forms being obtainable by extrusion, pelletization, spraying and other agglomeration techniques known by a person skilled in the art. These agglomerates do not have the inherent drawbacks of the pulverulent materials.
Moreover, the zeolite crystals are most often prepared from aqueous soda solutions (for example aqueous solution of sodium hydroxide), and, if desired, the sodium cations may be replaced (exchanged) wholly or partly with other cations, for example barium or barium and potassium. These cationic exchanges may be carried out before and/or after agglomeration of the pulverulent zeolite with the agglomeration binder, by conventional techniques known by a person skilled in the art.
The agglomerates, whether they are in the form of platelets, beads, extrudates, and others, generally consist of crystals of zeolite(s), which constitute the active element (in the sense of adsorption), and an agglomeration binder. This agglomeration binder is intended to provide cohesion of the crystals with one another in the agglomerated structure, but must also be able to endow said agglomerates with sufficient mechanical strength so as to avoid, or at the very least minimize as far as possible, the risks of fractures, splintering or breaks that might occur in industrial uses, during which the agglomerates are subjected to many stresses, such as vibrations, large and/or frequent pressure changes, movements etc.
These agglomerates are prepared for example by forming a paste of zeolite crystals in powder form with a clay slip, in proportions of the order of 80 to 90 wt % of zeolite powder to 20 to 10 wt % of binder, then forming as beads, platelets or extrudates, and thermal treatment at high temperature for baking the clay and reactivation of the zeolite, moreover the cationic exchange(s), for example exchange with barium, may be carried out before and/or after agglomeration of the pulverulent zeolite with the binder.
Zeolite bodies are obtained whose grain size is of some millimetres, or even of the order of a millimetre, and which, if selection of the agglomeration binder and granulation are done according to standard procedures, have a satisfactory set of properties, in particular of porosity, mechanical strength, and abrasion resistance. However, the adsorption properties of these agglomerates are of course reduced relative to the starting active powder owing to the presence of the agglomeration binder, which is inert with respect to adsorption.
Various means have already been proposed for overcoming this drawback of the agglomeration binder being inert as regards adsorption performance, including transformation of all or at least a proportion of the agglomeration binder into zeolite that is active from the standpoint of adsorption. This operation is now well known by a person skilled in the art, for example by the name “zeolitization”. So that this operation can be performed easily, zeolitizable binders are used, most often belonging to the kaolinite family, and preferably calcined beforehand at temperatures generally between 500° C. and 700° C.
Patent application FR2789914 describes a process for manufacturing agglomerates of zeolite X, of Si/Al atomic ratio between 1.15 and 1.5, exchanged with barium and optionally with potassium, agglomerating crystals of zeolite X with a binder, a source of silica and carboxymethylcellulose, and then zeolitizing the binder by immersing the agglomerate in an alkaline solution. After exchange of the cations of the zeolite with barium (and optionally potassium) ions and activation, the agglomerates thus obtained have, from the standpoint of the adsorption of para-xylene contained in C8 aromatic cuts, improved properties relative to adsorbents prepared from the same amount of zeolite X and binder, but whose binder has not been zeolitized.
In addition to high adsorption capacity and good properties of selectivity for the species to be separated from the reaction mixture, the adsorbent must have good properties of mass transfer in order to guarantee a sufficient number of theoretical plates for performing effective separation of the species in the mixture, as stated by Ruthven in the work with the title “Principles of Adsorption and Adsorption Processes”, John Wiley & Sons, (1984), pages 326 and 407. Ruthven states (ibid., page 243) that, in the case of an agglomerated adsorbent, the total mass transfer depends on the sum of the intracrystalline diffusion resistance and the diffusion resistance between the crystals.
The intracrystalline diffusion resistance is proportional to the square of the diameters of the crystals and inversely proportional to the intracrystalline diffusivity of the molecules to be separated.
The diffusion resistance between the crystals (also called “macroporous resistance”), for its part, is proportional to the square of the diameters of the agglomerates, inversely proportional to the porosity contained in the macropores and mesopores (i.e. the pores with width larger than 2 nm) within the agglomerate, and inversely proportional to the diffusivity of the molecules to be separated in this porosity.
The size of the agglomerates is an important parameter when the adsorbent is used in an industrial application, as it determines the head loss within the industrial unit and the uniformity of filling. The agglomerates must therefore have a narrow granulometric distribution, centred on number-average diameters typically between 0.40 mm and 0.65 mm in order to avoid excessive head losses. The porosity contained in the macropores and mesopores does not contribute to the adsorption capacity. Consequently, a person skilled in the art will not try to increase it with the aim of reducing the macroporous diffusion resistance, knowing that this would be to the detriment of the volumetric adsorption capacity.
To estimate the improvement in transfer kinetics, it is possible to use the plate theory described by Ruthven in “Principles of Adsorption and Adsorption Processes”, ibid, pages 248-250. This approach is based on the representation of a column by a finite number of ideally stirred hypothetical reactors (theoretical stages). The height equivalent to a theoretical plate is a direct measure of the axial dispersion and of the resistance to mass transfer of the system.
For a given zeolitic structure, a given size of agglomerate and a given operating temperature, the diffusivities are fixed, and one way of improving the mass transfer consists of reducing the diameter of the crystals. A gain in total mass transfer will thus be obtained by reducing the size of the crystals.
A person skilled in the art will therefore try to reduce the diameter of the zeolite crystals as much as possible in order to improve mass transfer.
Patent CN1267185C thus claims adsorbents containing 90% to 95% of zeolite BaX or BaKX for separating para-xylene, in which the crystals of zeolite X have a size between 0.1 μm and 0.4 μm, in order to improve the mass transfer performance. Moreover, patent US20090326308 describes a method for separating xylene isomers, the performance of which was improved by using adsorbents based on crystals of zeolite X smaller than 0.5 μm.
The applicant has observed, however, that the synthesis, filtration, manipulation and agglomeration of zeolite crystals smaller than 0.5 μm employ methods that are arduous, rather uneconomical and therefore difficult to apply industrially.
Moreover, such agglomerates comprising crystals smaller than 0.5 μm also prove to be more fragile, and so it becomes necessary to increase the level of agglomeration binder in order to strengthen the cohesion of the crystals with one another within the agglomerate. However, increasing the level of agglomeration binder leads to densification of the agglomerates, causing an increase in the macroporous diffusion resistance. Thus, although the intracrystalline diffusion resistance is reduced owing to the decrease in size of the crystals, the increase in the macroporous diffusion resistance as a result of the densification of the agglomerate does not allow an improvement in overall transfer.
Consequently there is still a need for zeolitic adsorbent materials prepared from FAU type zeolite that is easy to handle in an industrial context, i.e. whose constituent crystalline elements (or more simply “crystals”) are advantageously larger than 0.5 μm, but displays a total mass transfer that is improved relative to an adsorbent prepared from conventional zeolite crystals of the FAU type of identical size (i.e. above 0.5 μm), while still having a high adsorption capacity.
These improved adsorbents would thus be particularly suitable for gas-phase or liquid-phase separation of xylene isomers.